1. Field of the Invention
The present invention relates to a control valve used in a flow control assembly such as a mass flow controller.
2. Description of the Prior Art
A control valve, in which a valve body, capable of opening and closing a valve mouth, is driven by a strain force of a piezostack, has been practically used.
FIG. 6 roughly shows a holding or mounting construction for a piezostack in the conventional control valve. Referring to FIG. 6, reference numeral 61 designates schematically a piezostack comprising a plurality of piezoelements laminated together. The piezostack 61 is housed in a valve case 63 standing on an upper surface of a body block 62 of the control valve. Reference numeral 64 designates an adjusting nut screwed on an upper portion of the valve case 63 for fixing an upper fixed portion 61A of the piezostack 61. Reference numeral 65 designates a valve body provided so as to be engaged with a lower surface of a lower movable portion 61B of the piezostack 61. The valve body 65 is elastically held by means of for example a diaphragm 66.
Upon applying an appointed direct current voltage to the piezostack 61, the respective piezoelements of the piezostack 61 are strained or activated to dislocate the lower movable portion 61B downward and thus descend the valve body 65 by a pressing force, thereby adjusting an openness of a valve mouth 67. In addition, reference numeral 68 designates an orifice block and reference numeral 69 designates a fluid passage.
However, in the control valve in which the valve body 65 is driven by the strain force of the piezostack 61 in the above described manner, it has been absolutely required to prevent any movement other than a strain resulting from an application of voltage during the time when a position of the piezostack 61 is being set as well as after setting the position of the piezostack 61. But, the above described prior art exhibits the following disadvantages:
That is to say, in the above described piezostack 61, both the upper fixed portion 61A and the lower movable portion 61B of the piezostack 61 are formed on a horizontal flat surface so as to be engaged with a lower surface of the adjusting nut 64 and an upper surface of the valve body 65, respectively, so that, in the case where a parallelism of the upper fixed portion 61A and the lower movable portion 61B is not perfect or a verticality of the valve case 63 is not sufficient, as shown by an imaginary line for example adjacent portion 61B in FIG. 6, the lower movable portion 61B and the valve body 65 are so-called open-sided and thus a strain output of the piezostack 61 can not be perfectly transmitted to the valve body 65. As a result, the valve body cannot be operated in the set manner to hinder an appointed flow control according to circumstances. In addition, a set value is shifted by an external force, such as vibration, according to circumstances.